Cellulose ester films are employed as a transparent resin film (hereinafter also referred to simply as a film) which is employed as a polarizing plate protective film employed in liquid crystal displays. Since cellulose ester films are optically and physically beneficial as a polarizing plate protective film, they are commonly most widely employed.
Heretofore, cellulose ester films have been prepared in such a manner that dope is prepared by dissolving cellulose ester in halogen based solvents such as dichloromethane, and the resulting dope is cast from a casting die onto a casting support such as a drum or a belt (being a solution casting film production method). Conventionally, dichloromethane has been employed as a suitably solvent of cellulose ester films due to its advantage of being readily dried because of its relatively low boiling point (a boiling point of approximately 40° C.).
However, in view of recent environment protection, it is required that halogen based solvents such as dichloromethane are handled in sealed facilities. For example, countermeasures are to be taken which include a method in which leakage of halogen based solvents is prevented via an extensive closed system and even though the leakage would have occurred, any leaked halogen based solvents are adsorbed in the gas absorbing tower prior to discharge to the exterior. Further, methods are also employed in which, prior to discharge to the exterior, halogen based solvents are burned via heating, or decomposed via electron beams.
As mentioned above, the industrial solution casting film production method has resulted in a significant environmental load as well as the cost for solvent recovery. Consequently, various solvents other than halogen based solvents have been investigated, but no other solvents, which dissolve cellulose esters as targeted, have yet been discovered.
As a method to prevent the discharge of halogenated solvents to the atmosphere or to lower the solvent drying load of the solution casting film production method, Patent Document 1 discloses a polyester film production method based on a melt casting method using no solvents.
In the above film production method, easier melt film production is realized by lowering the melting point of cellulose esters by lengthening the carbon chain of the ester group. Specifically, melt film production is enabled by employing, as cellulose esters, those which are highly substituted with a propionate group and a butyrate group, each of which has a longer chain than an acetate group.
However, cellulose ester films prepared via the above method have resulted in drawbacks such as a decrease in mechanical strength and degradation of saponification properties due to high substitution with a propionate group or a butyrate group, each of which has a longer chain than an acetate group. When the cellulose ester film which suffers from such depredated characteristics is employed, during preparation of polarizing plates, close adhesion to polarizing plates degrades. Further, when employed upon being mounted on liquid crystal display devices, viewing field characteristics are markedly degraded due to changes in temperature and humidity, wherefore further improvements have been demanded.
Patent Document 1: Unexamined Japanese Patent Application Publication No. (hereinafter referred to as JP-A) 2005-178194